1. Field of the Invention
The present invention relates to Stirling Engines. More particularly, the invention relates to improvements in (1) maximum operating temperatures, (2) the regenerator to maximize performance, (3) a throttling system designed for low cost and maximum performance, (4) high pressure shaft sealing to allow external drives, (5) control of the displacer piston, (6) heat transfer tubing, and (7) a gas burner to provide heat to the engine.
2. Background Information
Stirling engine performance improvements are continually being sought to increase the benefit of these energy conversion devices and allow large scale commercial introduction into the marketplace. Cost reduction has also been a key research area for these engines due to their increased complexity over open cycle engines such as the Internal Combustion and Brayton engines which have achieved extensive commercialization success.
The maximum Stirling engine efficiency is related to the Carnot efficiency which is governed by the ratio of maximum working fluid temperature relative to the minimum fluid temperature. Improvements in technologies which increase the margin between the two temperature extremes is beneficial in terms of total cycle efficiency. The lower working fluid temperature is typically governed by the surrounding air or water temperature; which is used as a cooling source. The main area of improvements result from an increase in the maximum working temperature. The maximum temperature is governed by the materials which are used for typical Stirling engines. The materials, typically high strength Stainless Steel alloys, are exposed to both high temperature and high pressure. The high pressure is due to the Stirling engines requirement of obtaining useful power output for a given engine size. Stirling engines can operate between 50 to 200 atmospheres internal pressure; for high performance engines.
Since Stirling engines are closed cycle engines, heat must travel through the container materials to get into the working fluid. These materials typically are made as thin as possible to maximize the heat transfer rates. The combination of high pressures and temperatures has limited Stirling engine maximum temperatures to around 800.degree. C. Ceramic materials have been investigated as a technique to allow higher temperatures, however their brittleness and high cost have made them difficult to implement.
U.S. Pat. No. 5,611,201, to Houtman, shows an advanced Stirling engine based on Stainless Steel technology. This engine has the high temperature components exposed to the large pressure differential which limits the maximum temperature to the 800.degree. C. range. U.S. Pat. No. 5,388,410. to Momose et al., shows a series of tubes, labeled part number 22 a through d, exposed to the high temperatures and pressures. The maximum temperature is limited by the combined effects of the temperature and pressure on the heating tubes. U.S. Pat. No. 5,383,334 to Kaminishizono et al, again shows heater tubes, labeled part number 18, which are exposed to the large temperature and pressure differentials. U.S Pat No. 5,433,078, to Shin, also shows the heater tubes, labeled part number 1, exposed to the large temperature and pressure differentials. U.S Pat. No. 5,555,729, to Momose et al., uses a flattened tube geometry for the heater tubes, labeled part number 15, but is still exposed to the large temperature and pressure differential. The flat sides of the tube add additional stresses to the tubing walls. U.S Pat. No. 5,074,114, to Meijer et al., also shows the heater pipes exposed to high temperatures and pressures.
The next item, in the Stirling engines, which is critical to the maximum performance is the regenerator. This device must heat and cool the working fluid for each cycle of the engine which may be 20 to 100 times per second. The regenerators which have been typically used in the past have been mesh screen type regenerators. The regenerators are a very dense packing of fine mesh screens into layers which are hundreds of screens thick. The fine screens and multiple layers are required to transmit the heat at the very high rate requirements. These screen regenerators have significant pressure drop as the working fluid, typically Helium, Hydrogen, or Air, moves through the mesh at high speeds. The performance of the Stirling engine is thusly limited by the use of mesh screens. For very small Stirling engines a single annular slot has been used with success. The slot reduces the pressure drop but is limited by the amount of surface area in a single slot regenerator. U.S Pat. No. 5,388,410, to Momose et al., shows the mesh regenerator located inside the heating and cooling tubes, labeled part number 25. An improvement to this design is shown in this patent as part number 26. This patent uses a series of small annular pipes placed inside the heater pipe. The maximum heat transfer rate is limited by the minimum pipe diameter. The small tubes also touch each other on their exterior which blocks the working fluid flow.
Throttling of Stirling engines is typically accomplished by varying the amount of working fluid inside the engine. With this technique a significant amount of pumping and valving hardware is required to move the working fluid. This is complicated by the high working pressures which increases the size of the pumping hardware. A second technique to throttle the Stirling engine involves opening ports within the engine which are connected to dead volumes. This technique increases the total system volume which lowers the power but also results in a significant reduction in efficiency due the larger dead volume which the engine is exposed to for the entire piston stroke. U.S. Pat. Nos. 5,611,201 and 5,074,114 are unique in the use of a variable angle plate connected directly to each piston. Reducing the plate angle results in reduced movement of the piston resulting in reduced power levels. The throttling technique, using the plate angle, has the disadvantage of a higher system weight due to the large loads generated when converting the wobble motion of the plate to torque.
A further feature, which has been a significant problem for Stirling engines, is the sealing system. If a Stirling engine with a pressurized crankcase has an output shaft which is outside of the pressure shell it must deal with the sealing problem at the crankshaft. Working fluid leakage, at the seals, is a large problem for external shaft systems. The seal problem is overcome by placing a generator or pump inside of the Stirling engine housing. This technique eliminates the high pressure rotating seal. The rotating seal is easier to seal relative to a sliding seal. A pressurized crankcase eliminates the need for a perfect sliding seal but requires the rotating seal. The disadvantages to the high pressure seal include the high cost and potential requirement to replace working fluid in the engine. The high pressure seals have limited lifetimes which requires replacement of the seal.